Project Summary Optigenetic, gene, molecular, and stem cell therapeutic approaches attempt to treat blinding retinal degenerations by restoring function to individual retinal cells, in particular the photoreceptors. However, techniques used to assess photoreceptor function, such as visual acuity and retinal sensitivity, do not have single photoreceptor resolution. Our long-term goal for this project is to assess photoreceptor function at the same single-cell spatial scale with which we now approach treatments for blinding disease. Retinal imaging with adaptive optics (AO) has enabled noninvasive visualization of cone structure both in health and disease. Despite this, studies that assess cone function using AO remain sparse. Recently, our team demonstrated the ability to measure a stimulus-evoked functional signal arising from an intrinsic stimulus- evoked change in cone photoreceptor reflectance. Although we can now use AO to measure a functional signal from a population of cones, much remains to be learned. To what extent can the reflectance response be resolved to individual cones? How does the response correlate with other known physiological and psychophysical measures of visual function? What is the origin of the response? Answers to these questions will guide translation of the reflectance response into a biomarker capable of assessing photoreceptor function, dysfunction, and response to treatment. This study will: 1) Determine the extent to which a reflectance response can be measured in individual cones; 2) Establish reflectance response norms in a population of healthy controls; and 3) Clarify the origin of the intrinsic reflectance response. We will use multi-channel adaptive optics scanning light ophthalmoscopy (AOSLO) to deliver visual stimuli to the retina while simultaneously imaging the cone photoreceptors with near infrared light. We will optimize our acquisition and analysis parameters to maximize the reflectance response and extend our protocols to extract responses from individual cones. We will investigate how biological variables including retinal eccentricity, age, sex, and race impact the reflectance response and we will correlate the reflectance response with other measures of visual function. Finally, we will investigate the effects of imaging source coherence length, non-confocal AOSLO image detection modalities, and circadian rhythm on the reflectance response. The outcome of this work will be a fully characterized reflectance response biomarker for assessing photoreceptor function at a resolution at or approaching that of individual cones. Access to such a biomarker will impact the development and testing of novel retinal therapeutics aimed at restoring photoreceptor function.